Nitrogen-containing Group III-V compound semiconductors (Group III nitride semiconductors) have a band-gap energy that corresponds to the energy of light of infrared to ultraviolet wavelengths. This makes Group III nitride semiconductors useful materials for light-emitting elements that emit light of infrared to ultraviolet wavelengths and for light-receiving elements that receive light of infrared to ultraviolet wavelengths.
Group III nitride semiconductors are composed of atoms bonded together by strong atomic forces and have a high dielectric breakdown voltage and a large saturated electron velocity. These make Group III nitride semiconductors useful materials for electronic devices such as high-temperature-resistant and high-power radiofrequency transistors, too. Practically harmless to the environment, furthermore, Group III nitride semiconductors have been receiving attention as easy-to-handle materials.
A nitride semiconductor light-emitting element in which such a Group III nitride semiconductor is used typically has a quantum-well light-emitting layer. When voltage is applied to the nitride semiconductor light-emitting element, electrons and holes are recombined in a well layer as a component of the light-emitting layer and generate light. The light-emitting layer may have the Single Quantum Well (SQW) structure, or may alternatively have the Multiple Quantum Well (MQW) structure, in which well layers are stacked alternately with barrier layers.
The well layers in the light-emitting layer are usually InGaN layers, and the barrier layers are usually GaN layers. The resulting device is, for example, a blue LED (Light Emitting Diode) having a peak emission wavelength of approximately 450 nm, and this blue LED can be combined with a phosphor to form a white LED. When the barrier layers are AlGaN layers, the increased difference in band-gap energy between the barrier and well layers will lead to enhanced luminous efficiency. AlGaN, however, is difficult to grow into crystals with good quality compared with GaN.
Typical N-type nitride semiconductor layers used in nitride semiconductor light-emitting elements are GaN and InGaN layers.
For example, the nitride semiconductor light-emitting element described in Japanese Unexamined Patent Application Publication No. 11-214746 (PTL 1) has, between a substrate and a light-emitting layer, a first nitride semiconductor layer having an n-type impurity of 1×1017 cm−3 or less, a second nitride semiconductor layer having an n-type impurity of 3×1018 cm−3 or less, and a third nitride semiconductor layer having an n-type impurity of 1×1017 cm−3 or less, with the first one closest to the substrate. According to PTL 1, the low n-type impurity concentrations of the first and third layers make these layers highly crystalline underlying layers, and the good crystallinity of the first layer helps the second layer, which has a higher n-type impurity concentration, grow with good crystallinity on the first layer.
Japanese Unexamined Patent Application Publication No. 11-330554 (PTL 2) describes a nitride semiconductor light-emitting element that has a light-emitting layer between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer. In this nitride semiconductor light-emitting element, the n-type nitride semiconductor layer is an n-type multilayer film layer that is a stack of an In-containing first nitride semiconductor layer and a second nitride semiconductor layer whose composition is different from that of the first nitride semiconductor layer. At least one of the first and second nitride semiconductor layers has a thickness of 100 Angstroms or less. According to PTL 2, high crystallinity of the light-emitting layer, gained as a result of the superlattice structure of the n-type multilayer film layer in particular, improves the efficiency of the nitride semiconductor light-emitting element.
The nitride semiconductor device described in Japanese Unexamined Patent Application Publication No. 10-126006 has a first nitride semiconductor layer on and in contact with at least one side of a light-emitting layer. The first nitride semiconductor layer has a greater band-gap energy than the light-emitting layer, and second and third nitride semiconductor layers are provided on the first nitride semiconductor layer. The second nitride semiconductor layer has a smaller band-gap energy than the first nitride semiconductor layer, and the third nitride semiconductor layer has a greater band-gap energy than the second nitride semiconductor layer. According to PTL 3, the invention provides a nitride semiconductor device with high luminous efficiency.
Another disclosed structure is aimed at improving optical power and reducing leakage current and includes V-pits created in an upper portion of an n-type nitride semiconductor layer. The V-pits are carried over to an active layer and closed by a p-type nitride semiconductor layer. The importance is on a structure of the n-type nitride semiconductor layer and a formation method that provide desirable V-pits.
Japanese Patent No. 3904709 (PTL 4) discloses a structure that includes an “n-type In0.1Ga0.9N/In0.02Ga0.98N multiple quantum well adjacent layer (Si-doped, 5×1017 cm−3; well width, 2 nm; barrier width, 4 nm; 20 layers),” an “In0.2Ga0.8N/In0.05Ga0.95N multiple quantum well active layer (undoped; well width, 2 nm; barrier width, 4 nm; 10 layers)” thereon, and a “p-type GaN adjacent layer (Mg-doped, 5×1017 cm−3; 0.1 μm) thereon. The multiple quantum well adjacent layer has pits. The pits are carried over to the multiple quantum well active layer above and closed by the p-type GaN adjacent layer. The multiple quantum well adjacent layer has a structure now commonly referred to as the superlattice structure after its configuration.
Japanese Patent No. 3612985 (PTL 5) discloses that forming a 0.5-μm thick silicon-doped GaN layer (electron concentration, 1×1018/cm3) as a “strain relief layer” under an active layer at relatively low temperatures generates many V-pits, with some in the active layer. According to PTL 5, this significantly improves the photoluminescence characteristics of the active layer.
Japanese Patent No. 5415756 (PTL 6) and Japanese Patent No. 5603366 (PTL 7) disclose structures that have a superlattice layer referred to as a pit opening layer under an active region (active layer). Quantum well layers and a hole injection layer extend into pits originating from threading dislocations, and the pits are closed by a p-type contact layer. According to PTL 6 and 7, this improves luminous and wall-plug efficiency.